From: Robert Findley <rfindley@google.com>
Date: Fri, 29 Oct 2021 22:06:15 +0000 (-0400)
Subject: go/types: slice exprs to accept type sets with single underlying types
X-Git-Tag: go1.18beta1~665
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=71e6ab8f958bf346f9e0b46c4a35310615360215;p=gostls13.git

go/types: slice exprs to accept type sets with single underlying types

This is a port of CL 357779 to go/types. A test error message was
repositioned on the sliced operand.

Change-Id: Ie775c128f70d9cb08a2eba54b8bc082134ec3200
Reviewed-on: https://go-review.googlesource.com/c/go/+/359876
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
---

diff --git a/src/go/types/index.go b/src/go/types/index.go
index 613f6292e4..24c1812039 100644
--- a/src/go/types/index.go
+++ b/src/go/types/index.go
@@ -207,9 +207,14 @@ func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
 
 	valid := false
 	length := int64(-1) // valid if >= 0
-	switch typ := optype(x.typ).(type) {
+	switch u := singleUnder(x.typ).(type) {
+	case nil:
+		check.errorf(x, _NonSliceableOperand, "cannot slice %s: type set has no single underlying type", x)
+		x.mode = invalid
+		return
+
 	case *Basic:
-		if isString(typ) {
+		if isString(u) {
 			if e.Slice3 {
 				check.invalidOp(x, _InvalidSliceExpr, "3-index slice of string")
 				x.mode = invalid
@@ -221,26 +226,26 @@ func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
 			}
 			// spec: "For untyped string operands the result
 			// is a non-constant value of type string."
-			if typ.kind == UntypedString {
+			if u.kind == UntypedString {
 				x.typ = Typ[String]
 			}
 		}
 
 	case *Array:
 		valid = true
-		length = typ.len
+		length = u.len
 		if x.mode != variable {
 			check.invalidOp(x, _NonSliceableOperand, "cannot slice %s (value not addressable)", x)
 			x.mode = invalid
 			return
 		}
-		x.typ = &Slice{elem: typ.elem}
+		x.typ = &Slice{elem: u.elem}
 
 	case *Pointer:
-		if typ := asArray(typ.base); typ != nil {
+		if u := asArray(u.base); u != nil {
 			valid = true
-			length = typ.len
-			x.typ = &Slice{elem: typ.elem}
+			length = u.len
+			x.typ = &Slice{elem: u.elem}
 		}
 
 	case *Slice:
diff --git a/src/go/types/testdata/check/typeparams.go2 b/src/go/types/testdata/check/typeparams.go2
index c4b0c212d2..3694b0ed00 100644
--- a/src/go/types/testdata/check/typeparams.go2
+++ b/src/go/types/testdata/check/typeparams.go2
@@ -123,6 +123,11 @@ func _[T interface{ ~[]byte }] (x T, i, j, k int) { var _ T = x[i:j:k] }
 func _[T interface{ ~string }] (x T, i, j, k int) { var _ T = x[i:j] }
 func _[T interface{ ~string }] (x T, i, j, k int) { var _ T = x /* ERROR 3-index slice of string */ [i:j:k] }
 
+type myByte1 []byte
+type myByte2 []byte
+func _[T interface{ []byte | myByte1 | myByte2 }] (x T, i, j, k int) { var _ T = x[i:j:k] }
+func _[T interface{ []byte | myByte1 | []int }] (x T, i, j, k int) { var _ T = x /* ERROR no single underlying type */ [i:j:k] }
+
 // len/cap built-ins
 
 func _[T any](x T) { _ = len(x /* ERROR invalid argument */ ) }